1. Field of the Invention
The present invention relates to a process for the recovery of five-membered ring dicarboxylic acid anhydrides from aqueous solutions of the corresponding dicarboxylic acids.
2. Description of the Prior Art
After catalytic air oxidation of o-xylene or feedstocks containing o-xylene at about 400.degree. C., the resulting phthalic anhydride is precipitated in the art by cooling of the reaction gas in a finned tube condenser. By scrubbing with water, practically all the organic substances are dissolved out of reaction gas, thus, the gas is largely freed from phthalic anhydride, whereby the anhydrides are hydrolyzed to carboxylic acids. The resulting aqueous solution is then treated, inter alia, with catalysts, e.g., thiourea and sulfur dioxide to rearrange the maleic acid obtained by degradation to slightly soluble fumaric acid. After its separation, the filtrate, according to the state of the art, is discharged as effluent. After pure o-xylene oxidation, the filtrate still contains 16 to 40 g/l of citraconic acid, 2 to 14 g/l of phthalic acid, 1 to 7 g/l of benzoic acid, 2 to 10 g/l of fumaric acid and small amounts of other substances.
For reasons relating to environmental protection and inasmuch as the effluent still contains an amount of valuable substances--especially citraconic acid--it would be extremely useful to attain a technically simple treatment process. After all, about 2 to 4 tons of citraconic acid are produced in the production of 1,000 tons of phthalic anhydride by oxidation of o-xylene.
Isolation of the acid by distilling off the water is not a plausible solution as it is far too expensive energertically and, hence, economically. Because of the volatility of the free citraconic acid with steam, alkali neutralization of the acid was previously necessary and, after evaporation, acidification was performed to be able to recover the mixture of free acid. DE-AS No. 15 93 246 describes another method. There citraconic anhydride is obtained from the aqueous solution of citraconic acid by azeotropic distillation of all the total water with benzene, toluene, chlorobenzene, dichloroethane, trichloroethane and/or hexane. DE-AS No. 15 93 546 proposes to distill off 80 to 90% of the solution water in vacuo at a bottom temperature below 80.degree. C., then together with the water, to distill over the citraconic acid as anhydride at a bottom temperature over 80.degree. C. and to separate the water to recover the oily, water-insoluble citraconic anhydride. Thus treatment of the aqueous citraconic acid solution by distilling off the water must be performed in a costly way with known processes. However, as an alternative, recovery of citraconic acid and the other acids contained in the fumaric acid effluent by extraction with suitable basic means should be theoretically possible. To be able to separate the water, the extracting agent and the salts from the extracting agent and carboxylic acid must be soluble in the water. A chemical reaction of the base with the acid should not be expected to occur.
M. I. Yakushkin (SU-PS No. 168 674) has reported the extraction of lower aliphatic monocarboxylic acids--such as formic, acetic, propionic and butyric acid--from aqueous solutions with trioctylamine. The water-insoluble acid salt was then isolated and heated to about 290.degree. C., whereby the free acid was split off and recovered by distillation.
It was then found that dicarboxylic acids, such as maleic acid, citraconic acid, itaconic acid, phthalic acid, etc, could be extracted from aqueous solutions with tri-n-octylamine. Thus, for example, fumaric acid effluent can be subjected to solvent extraction with this base to remove all acidic contents. After separation of the oily water-soluble layer, a suitable aromatic hydrocarbon is mixed as entrainer for water. Upon heating, the entrainer and its water azeotrope distill over starting from about 130.degree. C. Then, surprisingly, citraconic anhydride even at 105 to 110.degree. C./15 mbar and phthalic anhydride in vacuo at about 160.degree. C. can be separated by distillation, after some more benzoic acid had precipitated as an intermediate cut. By use of suitable aliphatic solvents, citraconic anhydride also in an heteroazeotropic mixture with the solvent and water can be stripped off from the effluent extract. After distillation, however, in both cases the amine in the bottom is partially decomposed.
The present inventors have found that, quite surprisingly, tertiary amines with branched primary aliphatic side chains in the 2-position, such as tri-(2-ethylhexyl)-amine, tri-(2-ethylbutyl)-amine, tri-(2-ethyldecyl)-amine, etc., have been found to be stable under the process conditions. All other reactions, including the extraction of the acids from the aqueous solution and the treatment of 1,2-dicarboxylic acids by distillation as anhydrides, can be performed with this class of substance as was done in the case of tri-n-octylamine. Unfortunately, however, in contrast with the more strongly basic, unbranched tertiary amines, complete extraction of the acid constituents of the fumaric acid effluent is not possible with these amines.
An advantage in using the above named branched amines is that the decomposition of the salts into their components under thermal load occurs at about 20.degree. to 30.degree. C. less than when the other tertiary amines are used. The release of the dicarboxylic acids occurring at lower temperatures is mild to the bases and increases the yield of recoverable anhydrides, if the subsequent distillation is performed at the lowest possible temperatures, and therefore, in a good technical vacuum.
An alternative milder method for treating the salt mixture can be effected, for example, by distilling off the citraconic anhydride under a vacuum, after distilling off the water/hydrocarbon azeotrope. Then an inert gas such as nitrogen, hydrogen, carbon monoxide, carbon dioxide, air, etc., is introduced at an elevated temperature under vacuum to the tertiary amine, whereby benzoic acid and phthalic anhydride are sublimated. When air is used, it is very advantageous to use tertiary amines with branched, aliphatic side groups in the 2-position, since this class of substances has unexpectedly proved itself to be considerably more stable to air oxidation than unbranched tertiary amines.
In a similar way, other five-membered ring dicarboxylic acid anhydrides, like the acids of the fumaric acid effluent, can be recovered from the aqueous solutions of their dicarboxylic acids. For example, succinic acid, itaconic acid and methylsuccinic acid can be so recovered as five-membered ring dicarboxylic acid anhydrides. However, the recovery of these five-membered ring compounds has been with very low yields and with inadequate purities.
Therefore, a need clearly exists for a process for recovering five-membered ring dicarboxylic acid anhydrides which provides excellent yields of the anhydrides.